1. Technical Field of the Invention
This invention relates to a lens driving device for driving lenses in a direction along their optical axes, and an imaging device for acquiring image data representing object light.
2. Description of the Related Art
It is now prevalent that an imaging device for acquiring a digital image of an object is incorporated in small-sized devices such as a cellular phone and PDA (Personal Digital Assistant). Since the small-sized device is equipped with the imaging device, the user can easily take a photograph at any time without labor of carrying a digital camera or video camera. These small-sized devices, which usually previously incorporate a data communication function using radio wave or infrared rays, has an advantage of being capable of instantaneously transmitting a picked up image to another cellular phone or a personal computer.
However, since the imaging device incorporated in these small-sized devices, such as the cellular phone, is much smaller than an ordinary digital camera, the size of its components such as a lens and CCD (Charge Coupled Device) and the space for housing these components are greatly limited. Thus, in order that these small-sized devices are used in place of the digital camera, they are insufficient in the imaging function and image quality of the image. Their use is limited to taking a picture or image not requiring image quality, e.g. taking the picture in place of making a note, or taking the picture for a waiting screen of the cellular phone.
In view of these circumstances, in recent years, a small-sized CCD with high a density of pixels and a small-sized lens with high contrast have been developed. Correspondingly, realization of high quality of the picture taken using the small-sized device such as the cellular phone or the PDA has advanced rapidly. As regards the fulfillment of an imaging function which is an unsolved problem, these small-sized devices are desired to incorporate an auto-focusing function and a zooming function which are usually incorporated in the digital camera.
The auto-focusing function and the zooming function can be realized by moving a plurality of lenses in a direction along their optical axis (hereinafter referred to as a back-and-forth direction) within the imaging device. In the digital camera or digital video camera, previously known lens driving techniques use rotation by a DC motor or stepping motor, and use contraction/extension of a piezoelectric element. Where these techniques are applied to the small-sized devices such as the cellular phone, from the viewpoint of downsizing the device and accuracy of movement control of the lenses, a technique of using a hollow stepping motor is preferred in which a hollow cylindrical rotor encircling the outer periphery of a lens mirror cylinder with lenses held is rotated by applying a pulse current to a stator encircling the outer periphery of the hollow rotor. As this lens driving technique using the hollow stepping motor, proposed is driving the lens mirror cylinder along the optical axis through a moving mechanism such as a cam mechanism located between the lens mirror cylinder and the rotor (see e.g. JP-A-56-147132, JP-A-59-109006 and JP-A-59-109007); moving the lens mirror cylinder by the rotor itself (see e.g. JP-A-60-415, JP-A-60-416 and JP-A-60-417); and integrating the lens mirror cylinder and the rotor (see e.g. JP-A-62-195615). Among these techniques, according to the techniques described in JP-A-60-415, JP-A-60-416 and JP-A-60-417, and JP-A-62-195615, provision of any specific moving mechanism is not required, thereby downsizing the entire device. However since the lens mirror cylinder is moved in the back-and-forth direction while being rotated, the image may shift due to eccentricity of the lenses. On the other hand, according to the techniques described in JP-A-56-147132, JP-A-59-109006 and JP-A-59-109007, since the rotating force of the rotor is converted into force in the back-end-force direction by the moving mechanism arranged between the rotor and lens mirror cylinder, by providing a rotation stopper for limiting the movement of the lenses in the rotating direction, the lenses can be moved in the back-and-forth direction without being rotated, thereby avoiding inconvenience due to the eccentricity of the lenses.
Further, the imaging device using the hollow stepping motor is constructed of a plurality of components such as a stator and a rotor which are housed within a strong enclosure capable of enduring external stress. By using such an enclosure, even when external stress is applied, inconveniences that the components hit each other and the rotation of the rotor rattles can be avoided, thereby permitting the lenses to be accurately driven.
However, the techniques described in the above Patent References have been accomplished so that they are applied to a digital camera having an ordinary size, or the like. Thus, if these techniques are applied, as they are, to the small-sized device much smaller than the digital camera, there occur inconveniences that the lenses are not driven or a sufficient space for housing the components cannot be assured. For example, in the techniques described in JP-A-56-14713.2, JP-A-59-109006 and JP-A-59-109007, it is difficult to assure the space for providing the rotation stopper.